Japanese Patent Application No. 2001-49776, filed on Feb. 26, 2001, and Japanese Patent Application No. 2002-48116, filed on Feb. 25, 2002, are hereby incorporated by reference in their entirety.
The present invention is related to a light emitting device using electroluminescence (EL), and related to a display device and an electronic instrument with employment of this light emitting device.
In EL light emitting elements using the electroluminescence (EL), since light emissions are carried out in an isotropic manner and thus, directivities of the EL light emitting elements are inappropriate, there are the following difficulties. That is, considering a specific direction of the EL light emitting element, intensity of light is lowered and emitted light cannot be utilized in high efficiencies.
The present invention may provide a light emitting device capable of utilizing light in a higher efficiency, while increasing intensity of light along a specific direction.
The present invention may provide a display device using the above-described light emitting device, and an electronic instrument with employment of the light emitting device
A light emitting device according to one aspect of the present invention, comprises:
a substrate; and
a light emitting element section formed over the substrate,
wherein the light emitting element section includes:
a first light emitting layer in which light is generated due to electroluminescence;
one pair of electrode layers used to apply an electric field to the first light emitting layer;
a second light emitting layer which absorbs light generated in the first light emitting layer and generates light having a longer wavelength than a wavelength of the absorbed light; and
one pair of dielectric multilayer films which are formed under and above the second light emitting layer, respectively,
wherein a wavelength range of light that is reflected by the pair of dielectric multilayer films includes a wavelength range of light generated in the second light emitting layer.
In accordance with this light emitting device, after the light generated in the first light emitting layer due to the electroluminescence is absorbed by the second light emitting layer, light having longer wavelength than that of the absorbed light is generated in the second light emitting layer. The light generated in the second light emitting layer emits through the dielectric multilayer films to the substrate. As a result, it is possible to obtain a light emitting device that can emit light efficiently due to light excitation.
Also, since the wavelength range of the light that is reflected by the dielectric multilayer films includes the wavelength range of the light generated in the second light emitting layer, the light generated in the second light emitting layer is shut up between the dielectric multilayer films, so that the light propagation in the direction intersecting with the surface of the substrate can be restricted. As a result, the light having a very narrow light emission spectral width in the direction intersecting with the surface of the substrate can be obtained efficiently. Further, the light generated in the second light emitting layer may be emitted in a direction intersecting with the substrate, so that a surface light emission may be achieved.
This light emitting device of the present invention may have the following features (1) to (10):
(1) A wavelength range of the light generated in the first light emitting layer and a wavelength range of the light absorbed by the second light emitting layer may at least partially overlap with each other. In this feature, the light generated in the first light emitting layer can be absorbed by the second light emitting layer efficiently.
(2) A wavelength at a highest emission intensity in a wavelength range of the light generated in the second light emitting layer may be longer than a wavelength at a highest emission intensity in a wavelength range of the light generated in the first light emitting layer.
(3) The pair of dielectric multilayer films may include a first dielectric multilayer film and a second dielectric multilayer film,
the first dielectric multilayer film maybe formed closer to the first light emitting layer than the second dielectric multilayer film and
a reflectance of the first dielectric multilayer film may be larger than a reflectance of the second dielectric multilayer film.
In this feature, the light generated in the second light emitting layer may be emitted from the second light emitting layer toward the substrate.
(4) The first light emitting layer may include an organic light emitting material which emits light by the electroluminescence,
(5) The second light emitting layer may include a host material and a dopant material,
the host material may absorb the light generated in the first light emitting layer so that an excited state occurs, and the dopant material may be excited due to a transfer of the excited state of the host material to the dopant material, and the excited dopant material may emit light having a longer wavelength than a wavelength of the light absorbed by the host material.
(6) The second light emitting layer may include an organic light emitting material, and
the organic light emitting material may absorb the light generated in the first light emitting layer so that the organic light emitting material is excited, and the excited organic light emitting material may emit light having a longer wavelength than a wavelength of the absorbed light,
(7) A propagation direction of the light emitted from the first light emitting layer may be substantially equal to a propagation direction of the light emitted from the second light emitting layer
(8) This light emitting device may further comprise an optical member which collects the light emitted from the first light emitting layer.
In this feature, since the optical member is formed, after the light emitted from the first light emitting layer is collected, the collected light may be entered into the second light emitting layer. As a result, the utilization efficiency of light may be improved.
In this case, the optical member may be formed between the first light emitting layer and the second light emitting layer.
In this case, the optical member may be a lens layer of refractive index distribution type.
(9) The second light emitting layer may include photonic crystal that restricts light propagated in a surface direction of the substrate.
In accordance with this feature, since the photonic crystal is formed in the second light emitting layer, the light propagates in the surface direction of the substrate can be controlled in the second light emitting layer. Furthermore, the light generated in the first light emitting layer can be utilized in a higher efficiency.
In this case, the surface direction of the substrate implies a direction parallel to a surface of the substrate on which the first and second light emitting layers, and the dielectric multilayer films are stacked.
In this case, a pitch of the photonic crystal may be defined based on a wavelength of the light generated in the second light emitting layer.
(10) The light emitting device may include a plurality of the light emitting element sections, and
the second light emitting layers of the emitting element sections may generate light having different wavelengthes.
In this case, the second light emitting layers may be formed in the same level.
In this case, the light emitting element sections may be separated from one another by a bank.
This light emitting device may be applied to a display device. This display device may be applied to various sorts of electronic instruments. This light emitting device may further be applied to various sorts of electronic instruments. A concrete example of the display device and a concrete example of these electronic instruments will be discussed later.
Next, part of materials which may be employed in the respective sections of the light emitting device of the present invention will now be described. Apparently, the described materials merely are part of the materials known in this technical field, other materials than the exemplified materials may be selected.
Firsts and Second Light Emitting Layers
To obtain light having a given wavelength, the materials of the first and second light emitting layers are selected from compounds known in this field. As the materials of the first and second light emitting layers, any one of organic compounds and inorganic compounds may be employed. However, it is desirable to select the organic compounds in view of varieties of material sorts and film forming characteristics thereof.
In this case, materials which are employed in the first and second light emitting layers are selected in such a manner that the wavelength of the light generated in the second light emitting layer is longer than the wavelength of the light in the first light emitting layer. The materials for the first and second light emitting layers maybe selected in such a manner that the wavelength of the light generated in the first light emitting layer at least partially overlaps with the wavelength of the light absorbed by the second light emitting layer. Furthermore, a material in which energy transition is completed at one stage may be employed.
For instance, the first light emitting layer may be formed by employing 8-hydroxyguinoline-aluminum (Alq) and triphenyl-diamine derivative (TPD). In this case, the material of xe2x80x9cAlqxe2x80x9d has a function as an electron transport layer and another function as a light emitting layer, whereas the material of TPD has a function as a hole transport layer.
Also, the second light emitting layer may be formed from, for example, a material of Alq into which DCM2 has been doped. In this case, the material of DCM2 has a function of a dopant material, whereas the material of Alq owns a function of a host material. Alternatively, the second light emitting layer may be formed by a material of perylene tetracarboxylic acid dianhydride (PTCDA) into which pentacene has been doped. In this alternative case, the material of pentacene has a function of the dopant material, whereas the material of PTCDA has a function of the host material.
Dielectric Multilayer Film
In the light emitting element section, the dielectric multilayer film has a structure that materials whose refractive indexes are different from each other are alternately stacked. As a stacked layer structure, such a structure that a silicon oxide layer (SiO2) and a silicon nitride layer (SiNx) are alternately stacked may be exemplified, for example. Also, the dielectric multilayer film may be formed by alternately stacking two layers selected from. for instance, TiO2, Ta2O5, MgF2, and ZnS.
Electrode Layer
As the cathode, an electron injection type metal having a small work function (for example, lower than, or equal to 4 eV), an alloy, an electrically conductive compound, and a mixture of these materials may be employed. As an electrode substance, the electrode substance disclosed in Japanese Laid-open Patent Application No 8-248276 (1996) may be employed, for instance.
As the anode, a metal having a large work function (for instance, higher than, or equal to 4 eV), an alloy, an electrically conductive compound, or a mixture made of these materials may be employed. In the case that an optically transparent material is used as the anode, transparent electrically conductive materials such as CuI, ITO, SnO2, and ZnO may be employed. To the contrary, when the transparent characteristic of the anode is not required, a metal such as gold may be employed.
Also, the respective layers constituting the light emitting device may be formed by using the method known in this technical field. For example, as to the respective layers of the light emitting device, suitable film forming methods may be selected, depending upon materials thereof. Specifically, the vapor deposition method, the spin coat method, the LB method, the ink-jet method, and the like may be used